In conventional fuel injection systems, the fuel injectors may be mechanically, hydraulically, or electrically actuated. In hydraulically-actuated systems, the pumping assembly which periodically causes fuel to be injected into the engine cylinders is hydraulically driven by pressurized actuating fluid which is selectively communicated to the pumping assembly by an electronically-controlled valve. One example of a hydraulically-actuated, electronically-controlled fuel injection system is disclosed in U.S. Pat. No. 5,121,730 to Ausman, et al.
In mechanically-actuated systems, the pumping assembly is mechanically coupled to a cam driven by the engine so that the pumping assembly is actuated in synchronism with the rotation of the cam. The precise timing and duration of injection is determined by an electronically-controlled valve associated with the pumping assembly. Typically, the electronically-controlled valve is a solenoid valve.
In multi-cylinder engines in which such fuel injection systems are incorporated, it is important for optimization of the engine performance and emissions that the fuel injection characteristics for each engine cylinder are the same. The fuel injection characteristics include when fuel injection begins, the duration of injection, and the quantity of fuel injected.
In the past, uniformity of the fuel injection characteristics from cylinder to cylinder has been accomplished by adjusting the response of the solenoid valve associated with each fuel injector once when the engine is manufactured. Such a solenoid valve includes an armature, a pole member, and a spring which biases the armature away from the pole member. When such a valve is energized, the force of the spring pre-load must be overcome before the armature is urged towards the pole member. The valve response has been adjusted by changing the pre-load of the spring, which can be accomplished by changing the initial compression of the spring with one or more relatively thin inserts or shims.
For example, when a relatively thick shim is used to increase the initial compression of the spring, the solenoid valve takes longer to operate since a greater spring force must be overcome. When a relatively thin shim is used to decrease the initial compression of the spring, the solenoid valve operates more quickly since the spring force which must be overcome is less.
The practice of tuning a solenoid valve by adding, removing and/or changing shims is tedious and time consuming. For each fuel injector in the engine, such a procedure may require assembly of the injector, testing of the injector, disassembly of the injector to add or remove a shim, reassembly of the injector, testing of the injector, etc. Because of the time and effort required to disassemble and reassemble the fuel injector to change a shim, the fuel injectors are adjusted until a minimum, but not an optimal, performance threshold is achieved.